1. Field of the Invention
The present invention is directed to a power amplifier of the type having multiple, switched output stages and containing a driver which operates the output stages. The present invention is also directed to the driver for such a power amplifier, and a method for operating such a power amplifier. The invention is preferably utilized in a gradient amplifier of a nuclear magnetic resonance tomography apparatus that has a number of series-connected switched output stages for driving of a gradient coil.
2. Description of the Prior Art
German OS 43 04 517, corresponding to U.S. Pat. No. 5,546,299, discloses a power supply for an inductive load wherein two switched output stages are connected in series at the output side. A control arrangement composed of two control modules generates drive signals for active switch elements of the output stages.
During operation, the output stages apply a voltage of several hundred Volts to the gradient coil in order to periodically build up and dismantle an exactly regulated current of, for example, 300 A. Given a number of coupled output stages, operating conditions are possible wherein an energy transfer occurs between the output stages.
When, for example, the predetermined shape of the current curve exhibits steep leading edges and flatter trailing edges and the output stages assume the functions of a basic output stage and a peak load output stage, it can occur that the peak load output stage is cut in only during the steep leading edges of the current. The energy output from the peak load output stage to the gradient coil is then exclusively returned to the basic load output stage during the flatter trailing current edges. This can lead to an excessive charging (over-voltage) of a charging capacitor in the basic load output stage, and thus can result in an impaired function of or damage to the gradient amplifier.
Such an undesired charging of an output stage is also possible when the output stages are differently driven (for example, supply output voltages of differing polarity), when they exhibit different properties (for example, with respect to clock frequency or intermediate circuit voltage) or when a component known as a booster is connected into the load circuit.
In order to avoid over-voltages, German OS 43 04 517 and U.S. Pat. No. 5,546,299 teach the use of discharge resistors as a first alternative, that are connected in parallel with the charging capacitors. The presence of such discharge resistors, however, causes the energy consumption to be increased and the waste heat of the output stages to be eliminated with a cooling arrangement is considerable. As a second alternative, these corresponding documents teach the use of switchable discharge devices (DC--DC converters) for effecting the energy distribution between the output stages. Such discharge devices, however, are complicated in terms of realization, particularly when more than two output stages are provided.